Oxide products of uranium have various utilities including a preferred utility as nuclear fuels for nuclear reactors.
The performance of the fuel elements, traditionally enriched uranium dioxide structures clad in a metal container, is crucial to the practical success of the nuclear reactor. Nuclear power generation has imposed severe requirements on the performance of fuel in nuclear reactors, especially on properties of grain size and density of the fuel. It has been demonstrated that fine grain uranium dioxide structures are more subject to creep than large grain uranium dioxide structures. It has also been discovered that the density of the uranium dioxide is a very important physical property influencing the performance of the fuel. In fabricated forms, uranium dioxide is a ceramic capable of compaction to give a structure of desired density and a low impurity level.
The enrichment of uranium customarily takes place through use of the compound uranium hexafluoride so that a process is required for converting the enriched uranium hexafluoride into enriched uranium dioxide in a form which can be readily fabricated to structures having a low fluoride content and a desired density and grain size.
One current practice for converting uranium hexafluoride to an oxide product of uranium, usually uranium dioxide, employs hydrolysis of uranium hexafluoride to give a solution of uranyl fluoride and hydrogen fluoride from which ammonium diuranate is precipitated by the addition of ammonia. After filtration the ammonium diuranate of high fluoride content is dissolved in nitric acid with fluoride decontamination of the resulting uranyl nitrate solution being accomplished by solvent extraction. From the resulting purified uranyl nitrate solution, ammonium diuranate is reprecipitated and then calcined to give U.sub.3 O.sub.8 which in turn is reduced with hydrogen to give uranium dioxide.
Attempts have been made to replace this involved, expensive ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride with a very successful method being described in U.S. Pat. No. 3,796,672 entitled "Process for Producing Uranium Dioxide Rich Compositions from Uranium Hexafluoride" which is hereby incorporated by reference. This patent is in the names of W. R. DeHollander and A. G. Dada and is assigned to the same assignee as the present invention. This patent discloses a process for the conversion of gaseous uranium hexafluoride to a uranium dioxide rich composition in the presence of an active flame in a reaction zone by separately introducing a gaseous reactant comprising a reducing gas and a gaseous reactant comprising a mixture of uranium hexafluoride and an oxygen-containing carrier gas. The reactants are separated by a shielding gas as introduced to the reaction zone. The shielding gas temporarily separates the gaseous reactants and temporarily prevents substantial mixing and reacting of the gaseous reactants until sufficient cross diffusion occurs. The practice of the process of U.S. Pat. No. 3,796,672 gives a uranium dioxide rich composition having particularly desirable properties and a gaseous atmosphere rich in reducing gas such as hydrogen. Since it is known that certain gaseous mixtures of a reducing gas such as hydrogen and air can be readily combustible and potentially explosive, it has been found desirable to convert any such gaseous mixture to its oxidized form during this process.
U.S. Pat. No. 3,790,493 entitled "Post Oxidation Process for Uranium Dioxide Rich Compositions" covers a process having the improvement of introducing an oxygen-containing gas as a third gaseous reactant at a time when the uranium hexafluoride conversion to the uranium dioxide rich composition is substantially complete. This results in oxidizing the uranium dioxide rich composition to a higher oxide of uranium with conversion of the residual reducing gas to its oxidized form. This patent in the names of Abdul G. Dada, W. R. DeHollander and Robert J. Sloat is assigned to the same assignee as the present invention and is hereby incorporated by reference.
Another very successful method of replacing the ammonium diuranate conversion process by gas phase reaction of uranium hexafluoride is described in copending U.S. patent application Ser. No. 663,274 entitled "Process for Producing Uranium Oxide Rich Compositions from Uranium Hexafluoride" which is hereby incorporated by reference. This patent application was filed Mar. 3, 1976 in the names of W. R. DeHollander and C. P. Fenimore and is assigned to the same assignee as the present invention. This process comprises the conversion of gaseous uranium hexafluoride to a uranium dioxide rich composition in the presence of an active flame in a reaction zone by separately introducing a first gaseous reactant comprising a mixture of uranium hexafluoride and a reducing carrier gas and a second gaseous reactant comprising an oxygen-containing gas. The reactants are separated by a shielding gas as introduced to the reaction zone. The shielding gas temporarily separates the gaseous reactants and temporarily prevents substantial mixing and reacting of the gaseous reactants. The flame occurring in the reaction zone is maintained away from contact with the inlet introducing the mixture to the reaction zone.
In operation of any of the processes of U.S. Pat. Nos. 3,796,672 and 3,790,493 and U.S. patent application Ser. No. 387,529, it is desirable to connect the reaction zone with a powder collection apparatus, such as the apparatus disclosed in copending U.S. patent application Ser. No. 396,874 filed Sept. 12, 1973. This application entitled "Apparatus and Method for Collecting Particulate Material from a Gas Stream" was filed in the names of W. R. DeHollander, R. J. Sloat, W. R. Becker and A. G. Dada and is hereby incorporated by reference. This application directs the particulate (uranium oxide) containing gas stream into conduits housing filters which separate the particulate material from the gas stream with exhausting of the gas stream. The particulate material is then blown from the filters by a high pressure back flushing with a gas stream for collection of the particulate material in a container connected to the conduit. In practice it has been found that the filters used in the apparatus for collecting the particulate material from a gas stream encounter very high temperatures such as temperatures in the range of 450.degree. to 500.degree. C. Porous metal filters such as Monel metal filters have been found to have limited life at such temperatures with a typical failure rate occurring between 200 and 400 hours of operation. Accordingly it is recognized that it would be desirable to minimize the temperature of the gases encountered by the filters in the apparatus for collecting particulate material from a gas stream, and this is one of the objectives of this invention. At reduced temperature, the operating life of the metal filters is greatly increased and the type of filters capable of being utilized in this apparatus is expanded.